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The Power MOSFET
Today with the vast number of technical achievements occurring around th
world, many discoveries are overshadowed or obscured by some that ma
appear more important to the general media. One such discovery of importance,
to the audiophile at least, is that of the power MOSFET device.
The MOSFET
The field effect transistor (FET) and then the MOSFET transistor have bee
around for a number of years, but only as a small signal-handling device, mostl
employed in radio tuners and communications equipment. The electrica
advantages of these have long been realised by manufacturers of hi-fi. If onl
they could be made to handle large amounts of power – what a benefit to th
audiophile.
The term power MOSFET describes a device capable of handling reasonabl
large amounts of electrical energy as an amplifier itself – hence power.
MOSFET stands for “Metal Oxide Silicon Field Effect Transistor”, this in tur
means that the device is constructed of Silicon. Similar to a transistor – but th
part that controls the power flow through the device is insulated from th
remainder of the device by a metal oxide insulating layer and the controlling o
the power is achieved by the development of an electrostatic field between th
controlling element and the conducting element.
In a transistor, the control of the power through the device is effected by th
application of a smaller, but nevertheless, significant amount of power to th
controlling element. Whereas in the power MOSFET, the control of the powe
through the device is affected by the application of a very small and ver
insignificant amount of power to the controlling element – in fact, only th
amount required to create a small electrostatic field. This makes the operation o
a power MOSFET similar to that of a valve.
Other Field
Effect
Devices
There are basically three types of power field effect device, they are: th
junction FET, the vertical FET and the power MOSFET, all of which wer
independently developed by three different hi-fi equipment manufacturers i
Japan and all were major technological breakthroughs in their own right.
The first of these was the junction FET, the second the vertical FET and lastly,
the power MOSFET. Although all these devices are vast improvements ove
power transistors, the junction FET and vertical FET cannot compare with th
power MOSFET, in terms of simplicity of the supporting driver stages an
power supply requirements.
The power MOSFET, though having similar characteristics to the valve, can b
divided into 2 types of polarities of device – P-channel an
N-channel. Broadly speaking only one of these types exists in valve operations.
This means that complementary power MOSFETs – P and N channel – can b
used in an audio output stage providing greater linearity of operation than can b
achieved with valves. In addition, further advantages over the valve includ
